It is said that, in future, automotive transport will look much more closely to, and rely more upon, electrically powered vehicles. Electric vehicles are essentially zero-emission vehicles, which is to say that they are non-polluting at the power consumption point, and it is intended that current shortcomings of electrical vehicles should be overcome as quickly as possible. Among those shortcomings is the fact that batteries to be used for electric vehicles may comprise as much as 20% to 30% of the weight of the vehicle. That means, in turn, that the power delivered by the batteries must be such that it can move not only the vehicle and its passengers, but the weight of the batteries themselves. Moreover, even in city and light highway traffic conditions, an electric vehicle must be expected to accelerate in a manner similar to a conventional vehicle powered by an internal combustion engine, which means either that there must be excess battery capacity on board the vehicle to provide fast current delivery for vehicle acceleration, or that the batteries must be designed with a high ratio of active material surface to volume so as to be able to provide high power densities. Such demands may be similar to cranking power requirements in ordinary vehicles, but the demands may occur much more frequently and be of longer duration in each instance.
Moreover, electrically powered vehicles must have batteries that are capable of being recharged very quickly. While battery recharging is outside the scope of this invention, it is evident that just as the batteries must be designed so as to be able to deliver high current, quickly, so also must they be designed to absorb high recharging current, quickly. Still further, the choice of batteries in various experimental and low-production electric vehicle programs currently under way throughout the world has demanded consideration of various battery types, operating at various voltage levels.
Among the battery types presently under consideration for use in electric vehicles are nickel-cadmium batteries, which however are very expensive, environmentally undesirable, and tend to self-discharge. Moreover, nickel-cadmium batteries may exhibit a memory effect, in that they must be substantially fully discharged before they are recharged; otherwise, such batteries may develop a rate-limited "memory" phenomenon whereby they tend to lose their usable rate delivery capabilities if they are recharged when only partially discharged.
Nickel-iron batteries are also being considered, but it must not be overlooked that they were used at least as early as the Second World War to power submarines when running under water. Moreover, nickel-iron batteries tend to produce hydrogen gas, requiring very special ventilation, and creating explosive conditions. Still further, they are very heavy.
Even rechargeable alkaline manganese dioxide batteries are now being considered for electric vehicles. Bundles of very small cells are being assembled into batteries so as to provide high current delivery and high current recharge acceptance characteristics, which overcomes a shortcoming of such batteries in commercial cell sizes because they are not generally capable of high current delivery or high current recharge. Batteries made from rechargeable alkaline manganese dioxide cells also overcome the shortcomings of similar nickel-cadmium batteries in that they do not have any memory effects, and are not nearly so likely to exhibit overheating characteristics, especially during any recharge which is not precisely controlled. However, rechargeable alkaline manganese dioxide batteries are expected to be quite expensive in terms of the power density capable of being produced, and to have unacceptable cycle life.
Sodium-sulphur batteries will provide about three times the range of lead/acid batteries having the same weight. However, sodium-sulphur batteries must be maintained at temperatures of at least 315.degree. C. in order to produce electricity, making them extremely hazardous in use. Moreover, to maintain sodium-sulphur batteries at those high temperatures requires extremely good insulation plus electrically powered heaters--which is wasteful of stored electrical energy in the battery, as well as making a requirement for external power when the vehicle is parked.
Accordingly, the automotive industry has generally decided to maintain its reliance on lead/acid batteries as a principal power source for electric vehicles, for a variety of reasons. First, lead/acid battery technology is well known and accepted by the public, since most automotive batteries are currently lead/acid batteries. Lead acid batteries are also well accepted as traction batteries for industrial vehicles such as fork lift trucks and the like, and for recreational vehicles such as golf carts. Moreover, lead/acid batteries can be designed to be essentially non-polluting if they are closed or sealed batteries operating in a gas recombinant mode. Such batteries require very little maintenance, if any. Still further, even the disposal of lead/acid batteries is less ecologically sensitive an issue than previously, because techniques now exist whereby substantially all lead in such batteries may be recovered and recycled. This reduces, if not virtually eliminates, landfill and ground water contamination problems.
Moreover, in general it is possible to design lead/acid batteries that are capable of delivering and receiving relatively high current densities, so as to meet the requirements of electric vehicle acceleration, regenerative braking, and fast charging. However, lead/acid batteries still have a number of problems to overcome, including the problems of energy density, and the unfortunate tendency of lead/acid batteries to lose active material from their plates--particularly at high current densities. If active material is lost from a plate, then obviously the current capacity of the battery will be reduced; and moreover, there exists a strong possibility that active material detaching from the plates may result in short-circuited cells.
All of the major automobile manufacturers of the world are devoting very substantial sums of money to the development of electric vehicles. Indeed, some of those electric vehicles are hybrids, which use small auxiliary power sources--such as a small gasoline engine--whose purpose is to charge the batteries of the vehicle--especially during long or extended trips taken by the vehicle. Unfortunately, at least in California, such vehicles may not be accepted after 1998 because it is mandated that after 1998 at least 2% of the vehicles sold in California by any manufacturer who sells more than 5,000 vehicles annually in that state must be zero-emission vehicles. That means that hybrid vehicles, while attractive, may only be a short-term solution towards fully electric vehicles--with the possible exception of hydrogen powered vehicles.
Regrettably, this once again raises the problem of the capability of lead/acid batteries to continually absorb deep discharge without degrading, especially over many cycles having rapid battery recharging.
Certain bipolar lead/acid batteries have been developed by Jet Propulsion Laboratory, in association with Johnson Controls Inc., particularly for the XA-100 and XA-200 hybrid electric vehicle projects. In a paper published in 1991 by Johnson Controls Inc., "The XA-200: Proposed Hybrid Electric Vehicle Using the Bipolar Lead/Acid Battery" by M. Eskra et al, bipolar lead/acid batteries are discussed. Hybrid vehicles incorporating the described batteries are driven using a three-phase AC induction motor and an inverter controller system, carried in the frame of a conventional automobile but replacing its engine. According to the Eskra et al paper, it was found in early experimentation that the lead/acid batteries needed to be optimized so as to operate under high power conditions, and that the lead/acid batteries were too large and too heavy. The battery requirements were for the battery to deliver 60 kW for 20 seconds, and to have about 7 kWh total energy capacity. The weight allowed is 365 Kg, thus providing for energy density of about 19.2 Whr/Kg. The battery voltage is now expected to be in the range of about 200 volts; and the cut-off voltage for the battery is set at 100 volts, after which the battery is recharged.
The Eskra et al paper was given at the ISATA conference, held in Florence, Italy, during May 1991. The paper describes a bipolar battery having positive and negative active surfaces that are mounted back-to-back on an electrically conductive substrate, with separators placed between the opposed electrode surfaces of opposite polarity. As with any bipolar battery, the current path is straight through the battery, and the voltage increases with each cell that the current passes through. The bipolar battery is made of a stack of bipolar plates, and the stack is created and sealed by thermally welding together the plastic edges that are formed on each of the bipolar plates. It must be noted that each plate consists of a reinforced plastic substrate, with two thin lead grids on either side of the substrate, and having active material pasted into the grids. Electric current is carried through the reinforced plastic substrate by physical connections that are made between the thin lead grids which are on either side. The battery operates in a sealed, gas recombinant mode, with oxygen that is produced at the positive electrode or positive side of each plate diffusing to the negative electrode or side of the adjacent plate, where it is reduced.
It should be noted, as well, that lead/acid batteries in keeping with the present invention may find usefulness in other circumstances than electrically powered vehicles. In particular, lead/acid batteries in keeping with the present invention will provide suitable high levels of energy density and power density, long cycle life and fast recharging rates, all of which are critical or particularly attractive for utilization of the batteries in electric vehicles. Moreover, because of those characteristics, it will also follow that for given parameters any lead/acid batteries of the present invention may have a smaller footprint or volume, lower weight, and longer float life, all of which are critical or at least desirable for circumstances such as standby power applications. Such uses may include uninterruptable power supplies, and other standby and/or critical load power systems.
It was noted above that the Eskra et al paper describes bipolar lead/acid batteries that are capable of providing energy densities of about 19.2 Whr/Kg. Bipolar lead/acid batteries in keeping with the present invention will provide at least three times that energy density--up to 110 Whr/Kg.
Indeed, a variety of related inventions that are all directed towards battery plates for lead/acid batteries are described or at least referenced, below. Several issues, therefore, to be discussed include the provision of battery plates for lead/acid batteries, wherein an increased exposed active surface area for each plate, with respect to its projected area, will be provided. Thus, battery plates in keeping with the present invention are more capable of providing high current rates, from which it follows that battery plates as provided herein will exhibit a high active material surface to volume ratio. Conversely, such battery plates provide a lower density per unit of active area of the battery plates. The exposed active surface area of the plate will be at least 150% of its projected area.
Reference will be made herein to the provision of battery plates whereby various surface treatment steps may be taken to provide for the increased active surface area. Thus, steps are described to provide the placement of lead, and the manner in which the lead surface may be worked or machined so as to provide an increased active surface area.
Moreover, the present discussion is also directed to methods whereby the positive side of bipolar battery plates may be provided by oxidation of a prepared lead surface. There is therefore a fully prepared positive plate surface that is provided, and from that it follows that a formed battery will be provided when opposed prepared negative lead surface, together with the appropriate confinement structures, electrolyte, etc., are assembled. Thus, a manufactured battery is immediately ready to be placed in service without the necessity for post-assembly formation.
Other aspects to be at least referenced below include the provision of cored battery plates for bipolar lead/acid batteries. By providing a cored battery plate, where the core may be such as titanium, copper, chromium steel, or even silver, the weight of the battery will be reduced.
Still further, the teachings of the present invention, as they may be applied to bipolar batteries, provide for batteries having high rate, and high energy capacity to volume ratio characteristics. Moreover, it will be evident that methods of surface preparation and methods of oxide formation discussed below, are applicable to bipolar plates.
The present invention provides battery plates for lead/acid batteries, and in particular it will provide bipolar plates for lead/acid batteries. Moreover, the present invention will provide bipolar battery plates which are quite thin as to their lead constituent, but which comprise little additional structure which would otherwise add weight without providing either structural integrity or current density capabilities. Of even greater significance is the fact that bipolar battery plates that are fabricated according to the methods of the present invention will not exhibit any tendency to delaminate.
It must not be overlooked that, in a bipolar battery, each plate has a negative side and a positive side--active layers of lead and lead oxide, respectively--which are electrically connected through the physical core which separates the two sides of the single plate. Previously, bipolar plates for lead/acid batteries generally comprised a polymer or other plastic, non-conductive core, through which conductive pins or rivets electrically connected the positive side of the bipolar plate to the negative side of the bipolar plate. Current flows through the plate from the negative side to the positive side, not out the plate at the edge thereof as in a conventional stacked battery cell. By the present invention, a core is provided that is highly electrically conductive and, as well, is thermally conductive so as to preclude the possibility of unwanted temperature gradients developing within the battery.
However, the metallic cores provided by the present invention must be such that there is absolutely no likelihood of any pin hole or other path developing through the core whereby electrolyte may be permitted to flow from one side of the plate to the other. Because of their bipolar structure, each plate in a bipolar battery defines a cell, with voltage in the battery being cumulative according to the number of cells that are placed in the battery. However, if a pin hole develops whereby electrolyte from one side of a plate is allowed to flow to the other side of the plate, then adjacent cells--the cells at either side of the plate--are electrically shorted because the electrolyte in those two cells would then be in equilibrium. It is therefore important that the metallic cores that are provided by the present invention are structurally integral in all conditions.
When a bipolar plate is assembled and put into use, then there is a possibility that, especially on recharge conditions, gas may be produced within the cell. Each cell of a bipolar battery which is assembled according to the present invention should be vented. However, because bipolar batteries in keeping with the present invention are assembled using moldable polymer edge frames on each plate, the provision of a vent for each cell is easily accomplished, as is described hereafter.